An absolute optical position-measuring device of this type is known, for example, from U.S. Pat. No. 5,235,181. The position-measuring device described therein has a material measure extending along a measurement direction and having an incremental graduation as well as an absolute code. A scanning unit is movable relative to the material measure along the measurement direction. The scanning unit has a light source, a scanning grating for optically scanning the incremental graduation, as well as a detector device. The detector device includes an incremental detector for generating incremental signals from the optical scanning of the incremental graduation, as well as an absolute detector for generating absolute signals from the optical scanning of the absolute code. The exact absolute position of the scanning unit relative to the material measure can be determined by combining the absolute signals and the incremental signals.
Such position-measuring devices are used, for example, in applications where, in a machine, the position of a movable machine part relative to a stationary machine part must be determined with high accuracy to enable a machine controller to accurately position these machine parts relative to each other. If these machines are, for example, machine tools, then the conditions under which the optical position-measuring device is operated may impair the proper functioning thereof. For example, it is possible that contaminants, such as coolant/lubricants or oil mists, may accumulate on optical components of the position-measuring device, in particular on the material measure, which may take the form of a glass scale. In the extreme case, this can result in failure of the position-measuring device. In order to prevent such contamination-related failures, various protective measures have been described for optical position-measuring devices to minimize the likelihood of occurrence of such contamination. Examples of such measures include the encapsulation of the material measure with a surrounding profiled member, flushing of the profiled member with compressed air, installing filters in compressed-air units, etc. However, contamination of optical components of the position-measuring device as a result of such influences, and impairment of the proper functioning of the position-measuring device cannot always be prevented in this way.